From Event: SPIE Organic Photonics + Electronics, 2015
We have investigated the mechanisms responsible for nonlinear optical processes occurring in azobenzene-doped blue phase liquid crystals (BPLC), which exhibit two thermodynamically stable BPs: BPI and BPII. In coherent two wave-mixing experiments, a slow (minutes) and a fast (few milliseconds) side diffractions are observed. The underlying mechanisms were disclosed by monitoring the dynamics of grating formation and relaxation as well as by some supplementary experiments. We found the photothermal indexing and dye/LC intermolecular torque leading to lattice distortion to be the dominant mechanisms for the observed nonlinear response in BPLC. Moreover, the response time of the nonlinear optical process varied with operating phase. The rise time of the thermal indexing process was in good agreement with our findings on the temperature dependence of BP refractive index: τ(ISO) > τ(BPI) > τ(BPII). The relaxation time of the torque-induced lattice distortion was analogue to its electrostriction counterpart: τ'(BPI) > τ'(BPII). In a separate experiment, lattice swelling with selective reflection of <110> direction changed from green to red was also observed. This was attributable to the isomerization-induced change in cholesteric pitch, which directly affects the lattice spacing. The phenomenon was confirmed by measuring the optical rotatory power of the BPLC.
Chun-Wei Chen, Iam Choon Khoo, Shuo Zhao, Tsung-Hsien Lin, and Tsung-Jui Ho, "Studies of the underlying mechanisms for optical nonlinearities of blue phase liquid crystals (Presentation Recording)," Proc. SPIE 9565, Liquid Crystals XIX, 956505 (Presented at SPIE Organic Photonics + Electronics: August 09, 2015; Published: 5 October 2015); https://doi.org/10.1117/12.2189490.4519371799001.
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